Last summer, Bill Marczak stumbled across a program that could spy on your iPhone’s contact list and messages—and even record your calls. Illuminating shadowy firms that sell spyware to corrupt governments across the globe, Marczak’s story reveals the new arena of cyber-warfare.

The former Cleveland Browns quarterback’s epic fall—from the hottest thing in football to N.F.L. washout—was agonizing for those who tried to prevent it. His inner circle describes the clash of talent and temptation.